Polished metal and a method of making the same



NOV. 23, 1943. c, L, us 2,334,698

POLISHED METALS AND METHODS OF MAKING THE SAME Filed July 9, 1938 3Sheets-Sheet l Isnventor Char/es L. Fausf WWW (Ittomegs Nov. 23, 1943.c. L. FAUST 2,334,698

POLISHED METALS AND METHODS OF MAKING THE SAME Filed July 9, 1938 3Sheets-Sheet 2 Char/es L. Fauai F 5 (m mura Y J V Z Y? (Ittorne C. L.FAUST Nov. 23', 1943.

POLISHED METALS AND METHODS OF MAKING' THE SAME Filed July 9, 1938 3Sheets-Sheet 3 Zhwentor Our/es L. Fausfi at tomegs Patented Nov. 23,1943 POLISHED METAL AND A METHOD OF MAKING THE SALE Charles L. Faust,Columbus, Ohio, assiznor to Battelle Memorial Institute, Columbus, Ohio,a

corporation of Ohio Application July 9, 1938, Serial No. 218,388

6 Claims.

My invention relates to polished metals and a method of making the same.It is particularly applicable to stainless steel, although it is notnecessarlly limited thereto and may be applicable to various othermetals.

Metals are most commonly polished by mechanical rubbing and buiiing andthis practice has many drawbacks. For example, the mechanical polishingof stainless steel is one of the most expensive steps in itsmanufacture. As a result, polished stainless steel has hitherto soldfora price which is very high, when the cost of materials alone isconsidered. There are certain difliculties in rolling and other steps ofmanufacture that account, in part, for this high cost but, in the mainand particularly with certain types of objects, the mechanical polishingis one of the largest items in the total cost.

In the prior art, stainless steel has been polished mechanically onlyand the operation has not only been expensive but it has been diilicultbecause of the properties of the material. For instance, stainless steelis a poor conductor of heat and, in consequence, relatively high speedsof polishing tend to burn the surface thereof. This greatly limits thespeed of polishing. Furthermore, it is hard and tough and has a tendencyto pile so that the surface produced is cold Worked and strained.Likewise, commercially mechanically polished stainless steel is markedby microscopic scratches.

Manufacturers of stainless steel have wished vainly for other methods ofpolishing. Polishing by chemical agents has not been possibleheretofore. As a matter of fact, chemical attack by the more powerfulchemical reagents serves only to etch the surface and to develop a matteappearance of a type that is useless, so far as the demands of the tradefor a polished surface is concerned.

Some eifort has been made to clean ferrous alloys and other metals byelectrolytic treatment. However. these eliorts have been directed tocleaning only and definitely do not produce a polished surface. Suchsurfaces are sometimes described as etched, but it will be obvious thatan etched surface is not a polished one.

For example, one prior art patentee deals with an electrolytic processfor cleaning ferrous metals in preparation for nickel plating thereonand states that he obtains a product which is uniformly etched. But apolished surface of the type with which I am concerned would notnormally be covered up with nickel plating or other plating. Moreover,the type of polish with which I am concerned is, in the mai antitheticalto an etched surface.

Other prior art patentees have suggested the electrolytic treatment ofsuch metals as stainless steel for the purpose of removing or looseningscale preparatory to the subsequent treatment of the metal. But thesepatentees do not suggest that a polished or lustrous surface canbeproduced upon such materials by electrolytic treatment.

One object of my invention is to provide an effective method ofimparting such a high luster' to a metal or metal alloy that no furthertreatment will be necessary to render it highly ornamental and pleasingto the eye.

Another object of my invention is to provide a method oftreatingstainless steel which will produce thereon a surface which willbe highly lustrous and free from the scratches and piled layerscharacteristic of mechanically polished surfaces of stainless steels.

Another object of this invention is to provide a method for polishingmetals which will be less expensive than the mechanical polishingthereof while, at the same time, producing surfaces thereon. havingrelatively superior characteristics.

Another object of this invention is to provide a metal having a surfacethereon which is highly lustrous.

Another object of this invention is to provide a stainless steel havinga surface thereon which is highly lustrous.

Other objects of my invention will appear as this description progressesand from the appended claims.

This application is a continuation in part of my application, Serial No.160,890, filed August 25, 1937.

Generally stated, my method consists of treating the metal in anelectrolyte bath of suitable composition by making it an anode andpassing a current of sufficient density and for a suiilcient length oftime to produce a high luster or polish on the metal. I have found thatwith my method, I can produce a lustrous surface that is, at least,equal in appearance to surfaces obtainable by mechanical polishing andthat is superior in other, characteristics, such as freedom from "pilingand scratches characteristic of mechanically pol ished surfaces, so thatall necessity for mechanical polishing is obviated while, at the sametime, superior results are obtained. As a matter of fact, my methodpermits me to obtain almost mirror-like surfaces that are very muchsuperior to the results obtainable by ordinary mechanical polishing. Theresults which I obtain transcend a mere electrolytic cleaning process inthat there a is produced a highly lustrous or polished surface. Thishigh polish and luster is an important feature of my invention andsharply distinguishes it from prior art methods used in the electrolyticcleaning of stainless steel. Y

More specifically, I have discovered a method by which the surfaces ofthe grains of the metal being treated may be rendered substantiallylevel and that this results in a highly lustrous surface. Thus, thetreatment of stainless steel by the method which I shall now decribemore in detail results in a surface free from any amorphous layer and ofextremely high luster.

In carrying out my process, I pro ide a suitable electrolyte and makethe stainless steel or other metal to be cleaned the anode of the cell.cathode may be made of an inert conducting material. The electrolyte maybe widely varied in composition, although not all electrolytes aresuitable. A great many substances may be used to produce the result.However, certain electrolytes are much more desirable than others andsome of these electrolytes will be enumerated hereinafter.

I prefer to use sulfuric and phosphoric acids in substantial amounts inthis electrolyte mainly because of their low cost. Sulfuric acid may,however, be used without phosphoric acid in conjunction with any one ofa wide variety of organic materials such as glycerol, some alcohols,

' ethylene glycol, ammoniated glycyrrhizin, glycyrrhiza extract orlicorice, thiourea and its substitution products. These, when combinedwith sulfuric acid in suitable concentration, produce electrolytes whichare efi'ective in producing lustrous surfaces. Most of the materials Ihave found useful when added to sulfuric acid belong to the class ofchemical compounds which are known to form complex ions with such ionsas iron, chromium, nickel, etc.

In choosing materials for the electrolytic bath, I have discoveredcertain principles that offer some guidance in the selection. Theformation of the highly polished and lustrous surface that is adistinguishing feature of my invention is undoubtedly associated withthe presence of a polarizing film over the surface of the metal duringthe process of anodic attack. The nature of this film is such thatselective attack of the various phases present in the alloy isminimized. Anodic dissolution apparently takes place at arelatively'high rate at high anodic polarization values. The result ofthe anodic solution of the metal under these conditions is to level thecrystal surfaces of the metal and to produce a smooth and mirror-likefinish. These conditions do not prevail in thesimple electrolyticcleaning treatments known to the prior art.

I have found that it is preferable to keep the water content of theelectrolytic bath relatively low. While I have been able to producepolishes on stainless steel with mixtures of sulfuric and phosphoricacids containing. as much as 50 per cent water I find that, in eneral,baths containing less water will polish at lower current densities.

The mixtures of sulfuric and phosphoric acids which are suitable forcarrying out my process cover a wide range of compositions. I prefer tohave the total acid in excess of 50 per cent and not over 90 per cent,the balance in any case being water or organic addition materials suchas I have enumerated previously. Sulfuric acid when supplemented byphosphoric acid should be in excess of 5 per cent to produce the bestresults; phosphoric acid is not essential to my process, but its usepermits polishing at lower current densities and I prefer to use it inexcess of 50 per cent of the total bath composition.

In order to obtain the results I desire in a reasonable length of time Iprefer to use relatively high current densities. With the preferredmixtures of sulfuric and phosphoric acids the process may be operatedwith anode current densities of from 50 to 1000 amperes per square foot.though it will be understood that lower current densities may be used,in which case a longer time'will be required. Higher current densitiesthan this range may also be used though such high current densities arenecessarily costly.

The length of time to effect the desired results will depend on thecurrent density used and to some extent on the nature of the initialsurface. Rough surfaces will obviously require a longer time to polishthan relatively smooth ones. The treatment will also depend to somedegree on the previous heat treatment of the steel. The condition of thecarbon in the steel affects the attack upon the grain boundaries and insome cases it is necessary to increase the current density.

As examples of electrolytic baths and the conditions under which theyproduce good polishes on stainless steel, I cite the following:

Bath No. 1

Parts of weight Sulfuric i 15 Phosphoric acid 63 Water 22 With this bathexcellent polishes are obtained on 18-8 (Cr-Ni), 25-12 (Cr-Ni) and 12 to18 per cent Cr and other straight chromium stainless steels when madethe anode therein at current densities of 50 amperes per square foot forone hour. At higher current densities proportionally less time will berequired.

Bath No.2 Parts of weight Sulfuric acid 60 Water 40 Ammoniatedglycyrrhizim 0.01 Methanol 0.55

This bath is particularly adapted to polishing of 18-8 type (Cr-Ni)stainless steel when it is made the anode therein at current densitie of2000 per square foot for 1% to 2 minutes. This time sufllces to producea very brilliant surface at a bath temperature of to F.

Bath No. 3

Parts of weight Sulfuric acid 50 Glycerol 40 Water 10 Bath N0. 4

Parts of weight Sulfuric acid 16 Phosphoric acid 13 Glycerol 56 Water 15This bath is very satisfactory for polishing 18 per cent Cr-typestainless steel at current densities of around 1000 amperes per squarefoot.

Bath N0. 5

Parts of weight Sulfuric a 60 Phosphoric acid 30 Water 10 In a bath ofthis composition excellent polishes were obtained on 18-8 (Cr-Ni) and24-12 (Cr-Ni) stainless steel at current densites of 30 amperes or moreper square foot.

It will be understood that the relative amounts of materials in thebaths cited as examples may be varied considerably.

With my process I am able to produce stainless steel having a surfacewhich is different from that produced by prior art methods of polishingthis material. All polished stainless steel articles hitherto availablecommercially show microscopic scratches on their surfaces which areproduced by the mechanical process of buffing and polishing. Thesurfaces produced by mechanical polishing are characterized further bythe presence of a layer of material which has been altered by thepolishing process. Electron diffraction studies which I have made on18-8 (Cr-Ni) .indicate this to be a surface of ferritic materialproduced, presumably, by the mechani cal working of the surface bypolishing. Surfaces produced by my method on the contrary are free fromthe scratches and altered layer which characterize these mechanicallypolished surfaces.

Surfaces produced by my method are also very different and greatlysuperior to those produced by any electrolytic cleaning or picklingtreatment hitherto used. The electrolytic treatments of stainless steelby the methods of the prior art develop matte surfaces which, whenexamined microscopically, show an attack of the crystal boundaries and astrong development of the crystalline structure of .the metal. Thesurfaces resulting from my method show no trace of the crystallinestructure when xamined microscopicaily or at most a faint suggestion ofthis structure. This feature of my method which results in a suppressionof the development of the crystal structure also helps to produce thehigh polish which I obtain.

Still further evidence of the novelty of surfaces produced by my methodis found in metallographic studies which I have made of stainless steelstreated by my process and by ordinary methods of mechanical polishing.These studies are exemplified in the accompanying photolithographswherein:

Figure 1 shows the result of nickel plating a mechanically polishedsurface of stainless steel, magnification 100 diameters.

Figure 2 shows the result of nickel plating a stainless steel surfacepolished by my method,

magnification 100 diameters.

Figure 3 shows a cross section through a nickel plate on surfaces ofstainless steel which have been polished mechanically, magnification 500diameters.

Figure 4 shows a cross section through a nickel. plate on surfacespolished by my method, magnification 500 diameters.

Figure 5 shows the etching action of glycerregia on the surface ofmechanically polished stainless steel, magnification 100 diameters.

Figure 6 shows the etching action of glycerregia on the surface ofstainless steel polished by my method, magnification 100 diameters.

Figure 7 is a diagrammatic sketch illustrating the type of surface whichis obtained by electrolytically polishing stainless steel in accordancewith my method.

Figure 8 is a diagrammatic sketch illustrating the type of surfaceobtained by mechanically polishing stainless steel and furtherillustrating the so-called amorphous layer on said surface which resultsfrom mechanical polishing.

Figure 9 is a diagrammatic sketch illustrating the type of surfaceobtained by etching stainless steel.

Figures 1, 2, 3 and 4 collectively show the difference between thesurfaces produced by my method and by methods of mechanical polishing.Figure 1 shows that nickel plated on to a metallographicali polishedspecimen of stainless steel shows no structure at ordinarymagnifications. This is what would be expected from the nature of thestainless steel surface produced by mechanical polishing. Such a surfacewill be characterized by the presence of a layer of amorphous or pseudoamorphous material produced by cold working the surface, and nickel,plated on to a layer of this nature, would not be expected to show astructure at ordinary magnifications, as is actually found to be thecase. I refer to the layer of worked material on the mechanicallypolished stainless steel as amorphous without meaning to imply that suchis its actual state since I am aware that much controversy has beenwaged over the actual nature of this layer of worked material.

Surfaces produced by my method are free from this amorphous layer. Thepolish produced by my process is characterized by the presence of levelsurfaces of the crystals forming the stainless steel. Since the latticeconstants of nickel are approximately those of austenite, isomorphism ispossible and nickel plated on to such a surface would be expected todeposit in such a way as to continue the crystalline structure alreadypresent and to make it more apparent. That this actually occurs is shownby Figures 2 and 4. Figure 2 shows the result of plating nickel on 18-8stainless steel polished by my method. The crystal structure present onthe stainless steel surface is sharply brought out by such a plate.

That the structure exhibited by the nickel plate isactually the crystalstructure of the underlying stainless steel is shown by Figure 4. Thisfigure is a cross section through the nickel plate and the stainlesssteel. The continuity of crystal growth from the surface to the plate isclearly shown in this photomicrograph. It will be noted that suchcontinuity of crystal growth is not shown in Figure 3, where the nickelhas deposited on the amorphous layer produced by mechanical polishins.

Not only is the metallic surface produced by my method different fromthat resulting from mechanically, polishing stainless steel, but thefilm covering the surfaces is also different. This is proven by Figures5 and 6 which show the difference in behavior of the two surfaces of18-8 Cr-Ni stainless steel when subjected to an etching treatment withglycer-regia. Mechanically polished surfaces are subject to attack whichimmediately develops the crystal structure of the material, as shown inFigure 5. Specimens polished by my method on the other hand areapparently covered by a film which permits attack only at discretepoints which results in a pin hole structure, as shown in Figure 6.Substantially similar results are obtained with straight chromiumstainless steels.

Referring to Figures '7, 8 and 9, which are sketches showing inexaggerated form the grain structures of stainless steel and the typesof surfaces produced thereon by my method, by mechanical polishing andby etching, it will be noted that the surface illustrated in Figure 7which results from electrolytic polishing by my method is one whereinthe surfaces or the grains are substantially level and are not coveredby any layer of amorphous material. noted that on mechanically polishedstainless steel the polished surface is alayer of amorphous material.In' Figure 9, it will be noted that the etched surface is made,up ofgrains whose surfaces are not level. This condition is characteristic efthe etched surfaces such as those produced by electrolytic cleaning.

From this description of my invention it will be apparent that I haveprovided a novel and effective way of producing a polish on stainlesssteel. My method avoids all the disadvantages of the old method ofmechanical polishing and allows me to produce stainless steel articleswith surfaces which are free from mechanical strain, dragging, andpiling, and which are betterthan those made by mechanical polishingmethods By the term stainless steel I-mean thoseInFigure8,itwillbesteels which are made corrosion-resistant by theaddition of such alloying elements as chromium or chromium and nickel.and which are exemplifled by the examples I have given.

Having thus described my invention, what I claim is:

l. The method of imparting a lustrous surface to stainless steel whichcomprises making such stainless steel the anode in'an electrolytic bathof substantially the composition: sulfuric acid, 15 per cent; phosphoricacid, 63 per cent: water, 22 per cent, and passing a currenttherethrough at an anode current density in excess of 50 amperes persquare foot for a period of time sufllcient to impart the desiredluster.

2. The method of imparting a lustrous surface to stainless steel whichcomprises making such stainless steel the anode in a solution of 5 to 85per cent sulfuric acid, 5 to 85 per cent phosphoric acid, the combinedacid concentration being between 50 and 90 per cent and the balancebeing substantially water, and passing a current or at least 30 amperesper square foot anode current density therethrough for a period of timesum- .cient to impart the desired luster to said surface.

3. The method of imparting a lustrous surface to stainless steel whichcomprises making such stainless steel the anode in an aqueouselectrolyte containing at least 5 per cent of sulfuric acid and at leastper cent of phosphoric acid, the total acid concentration being not over90% and the balance being largely water, and passing an electric currenttherethrough of sufficient density and for a sumcient period of time toefiect such lustrous surface.

4. The method of imparting a lustrous surface to stainless steel, whichcomprises maln'ng such stainless steel the anode in an aqueouselectrolyte containing at least 5% of sulfuric acid and at least 50% ofphosphoric acid, the total acid con-- centration being not over 90% byweight and the balance being largely water, and passing an electriccurrent therethrough of a density of from 30 to 2000 amps./sq. ft. forfrom 1% minutes to 1 hour, the time being shorter for the higher currentdensities and longer for the lower current densities while maintainingthe electrolyte at a temperature between and 1'15 1?.

' 5. A new article of manufacture having a stainless steel surface, saidsurface being anodically polished to remove metal therefrom and rendersaid surface free from mechanical strain and from amorphous layers andshowing substantially no crystalline structure microscopically.

6. The method of imparting a lustrous surface to stainless steel, whichcomprises making such steel the anode in an aqueous acid electrolyteconsisting initially of about eleven per cent water by-weight, sulfuricacid, and phosphoric acid, each of said acids being present insubstantial effective amounts, and passing an electric cur-' renttherethrough of sufficient density and for a suflicient period of timeto obtain such lustrous surface.

CHARLES L. FAUST.

